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Topic: Compression test values (Read 5889 times)

LJRead

I'm trying to get a feel for what to expect with regard to the values obtained with a pressure tester. The compression values for the Classic and AVL are 6.5:1 and 8.5:1respectively, but how would that translate if I were to place a compression testing gauge on it with a 0-300 psi range.?

You are mixing compression RATIOS with direct compression VALUE.Compression testers are to enable you to examine the state of the engine in relation to ring sealing, exhaust and inlet valve seating, decomp device seating etc. An 80PSI pressure applied from a dual gauge (ideally) compressor set-up with the engine set exactly on TDC (top dead centre) on the firing stroke (Ie. both valves closed) should show a small leakage. Now depending if air can be heard from exhaust/silencer then that'd signify a leaking exhaust valve etc.The spin over type gauge that just gets shoved into the spark plug hole while the engine gets turned over is less useful but will show ring leakage especially if the figure goes up when some oil is poured into the spark plug hole (effectively helping to seal rings etc)Compression RATIO is a value given to an engine on how much the piston compresses the air space above the piston. Very basic explanations of it all, but you get the drift???

I was going to say that with most gasoline motor(carsand bikes) that don't have hi compression pistons, the normal figure is usualy around 120psi. That seems to be the norm in my experience. Milled heads, high compression pistons will up that figure. Diesels are another story in HIGH compression. Once single cylinder motors gets down around 80psi it is rebuild time. On a multi cylinder motor the compression shoud not vary more than 5-10psi between cylinders. Hutch

LJRead

Hey, watch out - that "Avatar" is a Royal Enfield Forecar, and since R E made it you can bet that the compression values wouldn't change for a hundred and two years. It is well known that things never change with R. E.

There is a direct connection between compression ratios and the pressure thats read with a "gauge in the plug hole". Why wouldn't there be? Low ratio, low ultimate reading, High ratio, high reading. If this were not true how could a 18:1 diesel have 400lbs of compression? According to published tables,a healthy 6.5:1 should have110lbs, 7:1-120 7.5:1-130 8:1-140 and so on. Always remember the throttle is to be wide open, as it varies the inlet air and therefore the compression. Also the speed at which you spin the engine should be spirited, as say the idle speed. This gives a truer reading of actual running, and shortens the time of leaking, the condition that would exist if the motor was really running.

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LJRead

Yeah, of course if you compress 8.5 volumes of air space into one volume, it has a direct effect of pounds of pressure produced. In a site on compression testing in cars the value for an average car was about 140. I would expect about that for the AVL, or maybe slightly less. Surprised at the high compression ratio you mention for some diesels. Didn't realize they got up that high.

I paid a few bucks for a used compression tester, figuring that compression is a useful thing to keep track of. Sort of like a doctor measuring blood pressure, or better his pretty nurse (higher values!). But I guess high blood pressure in humans is a bit like very low pressure in bikes - shall we say symptomatic.

Compression ratio is the relationship between the "swept volume" (ie bore area x stroke) and what is often termed the "squish volume" (which is the volume left in the top of the cylinder when the piston is at top dead center).

compresssion ratio = squish/(squish + swept)

Domed pistons and skimmed heads reduce the volume of squish and therefore raise the compression ratio.

Compression pressure should be equal to air inlet pressure x compression ratio (+/-gains/losses).

Of course this is all subject to valve timing, pulsing in the inlet, friction, altitude and many other factors but that's the basics.

Getting a good baseline reading is probably a good idea and may be useful for troubleshooting in the future. Trouble is to get a good reading the engine should be at normal operating temperature, but when you're troubleshooting it's generally because the thing ain't running.

LJRead

Good point about obtaining baseline data. I can do it with a warm engine and a cold one, so if it isn't running when I need to trouble shoot, I'll have both values. I guess Hutch and others aren't far wrong with values of around 120 psi for an 8.5:1 ratio.

for most of you it's not an issue, but I live at about 5500ft so my normal cold-cranking pressure is about 95 psi. As a good rule of thumb, figure air density will drop about 3% per thousand feet of elevation.

for most of you it's not an issue, but I live at about 5500ft so my normal cold-cranking pressure is about 95 psi. As a good rule of thumb, figure air density will drop about 3% per thousand feet of elevation.

Wow, I never considered that. So when you come down to sea level it must be like adding a supercharger.